Control circuit for high speed selector



April 1940- w. H. T. HOLDEN 2,197,501

CONTROL CIRCUIT FOR HIGH SPEED SELECTOR I Filed April 23, 1938 FIG.

PHASE 2 PHASE INVENTOR W.H. 7THOLOEN ATTORNEY Patented Apr. 16, 1940UNITED STATES PATENT OFFICE CONTROL CIRCUIT FOR. HIGH SPEED SELE CTOB

York

Application April 23, 1938, Serial No. 203,756

15 Claims.

This invention relates to motor control devices and more particularly toan arrangement for controlling the direction of current fiow inalternating current light-duty motors, preferably of the polyphase typewhich requires sensitive startstop control.

More especially the present embodiment of the invention is adapted tothe control of the driving motor of a selector switch of the typedisclosed in an application of W. W. Carpenter, Serial No. 133,969,filed March 31, 1937, Pat. No. 2,123,228, granted July 12, 1938. Thisswitch is a highspeed selector of the panel type and comprises a singlebrush set which is driven by a steel tape in either direction totraverse two banks of terminals. The tape passes over a driving pulleyat the end of one bank of terminals and an idling pulley oppositelydisposed at the end of the other bank of terminals. An alternatingcurrent motor geared to the driving pulley provides the necessary powerto pull the tape in either direction while a brake associated with itnormally holds the tape against movement when the brush set is incontact with a set of terminals.

This switch is capable of traveling at very high speeds, hunting speedsof 250 terminals per second being easily attainable and hunting speedsof 400 terminals per second being possible. It becomes necessary,therefore, that, in order to to insure that the brush set shall bequickly stopped and accurately centered on the terminal set which hasbeen marked with av calling condition, a particularly sensitive controlcircuit be provided which is adapted to respond quickly to the testcondition on the marked terminal, to open the circult of the motor inresponse thereto, to apply it again in the reverse direction if thebrush overrides the marked terminal, and to repeat these operations inboth directions of travel as so often as necessary to accurately centerthe brush set on the terminal set with which it is to be connected.

One of the objects of the invention, as adapted to the operation andcontrol of the above-described type of switch, is, therefore, to providemeans for applying current of proper phase relationship to the motor ona signal that a line terminating in either bank of the switch iscalling, for supplying power to the brake magnet to release the drivingtape when the switch is about to move, for shutting off current to themotor when the brush set contacts with the terminals of the callingline, and for reapplying the current in alternately reverse directionsas often as necessary if the brush set rides beyond the wanted terminalwhile attempting to stop thereon or beyond the wanted terminal when thedirection of travel is reversed to bring the brush set into connectionwith said terminal.

To attain these ends, one feature of the present invention is a novelarrangement of four thyratron tubes rotatively arranged in pairs and soelectrically interconnected that each pair will control, in sequence,the movement of the selector in the opposite directions of travel. Thusthe first tube in each pair is responsive to a signal from a callingline in the bank in which the line appears, and the second tube isresponsive to the electrical condition imposed upon the test terminal ofthat line. Now the first tube, upon being ionized, controls a source ofpower which causes the motor to be operated in the appropriate directionto move the brush set over that bank of terminals in which the callingline appears, and, at the same time, primes the second tube of the pairin order that it may respond to the terminal signal when the brushconnects with the line terminal, an operation which, when it occurs,will have the efiect of extinguishing the first tube to stop the motorand of priming the first tube of the second pair of tubes in readinessto control the operation of the motor in the reverse direction shouldthe brush set ride over the terminal on which it is to come to rest andthus break contact with it. Should this come to pass, however, animpulse of current will be produced that will ionize the primed tube,the effect of which will be, first, to cause a reversal of current fiowin the motor to drive the brush set in the opposite direction toreengage the terminal and, second, to prime the next tube of the secondpair. When the brush set again engages the terminal, an impulse ofcurrent is again produced which ionizes the primed tube, extinguishesthe conducting tube whichthereby shuts ofi the power to the motor tostop the movement of the brush set, and further reprimes the succeedingtube of the first pair in readiness once more to apply power to themotor in the reverse direction should the selector, in advancing, failto stopv on the wanted terminal and thereby produce another impulse thatwill render the primed tube conducting.

Thus with an electrical guard provided which controls a thermionicdevice that will cause the selector to move in alternate directions,should the brush set overrun the terminal to be selected in eitherdirection of its movement, over-stepping is practically impossible andaccurate terminal selection is thereby insured.

trol circuit, of a full wave rectifier which provides current of properphase relationship to the motor on signal from the thermionic device,and further provides rectified current to the brake magnet of theselector at the time said magnet must disengage the brake clutch topermit the tape to pass freely over the driving and driven pulleys whenmoving in either direction of travel. This rectifiercomprises a pair ofthyratron tubes for'each direction of phase displacement of the motorcurrent, having paralleled grids which are connected to the cathodes ofthe alternate thyratrons of the control circuit to render theserectifying thyratrons conducting on thesignal produced by said alternatethyratrons when they become ionized either on the original signalproduced when a line initiates a call in either bank of theselector oron the succeeding signals produced by the test brush of the selectormaking and breaking the opposite edges, respectively, of the markedterminalin the operation of centering thereon. The cathodes of therectifying thyratrons are connected to a suitable source of power and,if they are of the "heater" type, to a suitable source of alternatingcurrent power, while the anodes are each connected to terminals ofseparate primary windings of a transformer whose other terminals extendto the first phase of the current supply, while the secondary windingsof said transformer are connected to a set of phase windings of theselector motor the other phase winding of which is con nected to thesecond phase of the current supply.

Before proceeding with the detailed description of the invention, itwould be desirable to mention afew of the properties of the thyratrgntube of which use is made both in the control circuit and rectifier.This tube is a three-element valve containing a cathode heated by afilament, a grid and an anode. The enclosing envelope, however, insteadof being completely evacuated, as would ordinarily be the case with theconventional vacuum tube, contains a trace of mercury vapor or someinert gas such. as argon, at low pressure, which, when ionized, reducesthe space charge and causes an arc to strike which, in reality, is but aheavy anode current that passes between the cathode and anode. The arecan be prevented from striking, however, if the grid potential is less,that is, less negative, than a certain critlcalvalue. In this way, asmall negative voltage on the grid can control a much larger positivevoltage on the anode. If, however, the grid be made very slightly morepositive than the critical negative voltage, the anode current startsimmediately and, once started, it continues until either the anodecircuit is broken for a fraction of a second or the potential dropacross the cathode and anode is reduced or reversed by some means for atime long enough for the positive ions which have formed around the gridto diffuse to the walls of the tube.

Having givena brief summary of the main features of the invention and ofthe control element which is used to attain them, we may now obtain aclearer conception of the scope and purpose of the invention from thefollowing detailed description of the operation of the invention, takenin connection with the appended claims and the attached drawing in whichFig. 1 shows the control circuit IIII, the full wave rectifier I and askeletonized form of the selector, briefly described above andchosen toillustrate the application of the invention, while Fig. 2 shows therelationship between one of the brushes of the brush set of the selectorand one of the terminals of the terminal set to be selected.

Referring to Fig. 1, reference character II! indicates the brush set ofthe switch, movable over the terminal banks I 2I and I22 by means of themetallic tape II6, driven by pulley III and passing over theidler pulleyH6. The tape is maintained in a state of sufficient tension by springsI6 I and I62 connecting its ends to the brush carriage. Pulley II! isdriven by the reversible polyphase induction motor II2 through shaftingI18. This motor has two sets of windings, the lower or main winding I19being supplied with current from transformer I06, which is connected toone phase of a sixty-cycle cur rent supply, and the auxiliary windingsI8I being supplied with current from winding 8-9 of transformer IIIwhich, in turn, receives the current from the second phase of thesixty-cycle power supply connected to the primary of transformer H9 atterminals I2 thereof. Thesecondary winding of transformer H9 is tappedat its midpoint I by conductor I and extends to the winding-of the brakemagnet I the other side of which extends to the cathodes of all therectifier thyratrons I06, I01, I06 and I09 which, when operating inpairs in the manner to be presently described, provide rectified currentto operate said brake magnet.

- It will be observed that the circuit of the main winding I19 of themotor is carried through a set of contacts on the brake magnet I05 and,therefore, this circuit will be ineffective until these contacts areclosed, which occurs, as will be presently explained, a short time afterthe current has been applied to the secondary windings I6I. The reasonfor this is, of course, to keep the main winding open-circuited so longas the motor is not in use.

Terminal 3 of the secondary winding of transformer II9 is'connected toterminal I of the primary winding-I2 of transformer I II and also toterminal I of the primary winding 6-I of the same transformer. Terminal2 of winding I-2 of transformer III is connected to the anode of tubeI06, while terminal 6 of winding 6-! of said transformer is connected tothe anode of tube I09. Terminal 5 of the secondary winding 4-5 oftransformer H9 is connected to terminal 4 of the primary winding 34-5 oftransformer III. Terminal 3 of this winding is connected to the anode oftube I0'I, while terminal 5 of this winding is connected to the anode oftube I08.

Let us suppose that the cathcde heaters of thyratrons I06, I01, I08 andI09 are heated to incandescence from the alternating current obtainedthrough the secondary winding 6-I of transformer II9, that the grids ofthe thyratrons I06 and I01 are at cathode potential or positive thereto,while those of thyratrons I08 and I09 are at a sufliciently negativepotential with respect to their cathodes to be non-conducting at anypoint in the voltage cycle. Assume, further, that, at a given instant,terminal 3 of the secondary of transformer I I9 is positive. Thencurrent will-flow thence to terminal I of the winding I-2 of transformerIII, through this winding to the terminal 2 thereof and thence to theanode of tube I06, in which'an arc will strike and current will fiow assoon as the anode reaches a certain potential value positive withrespect to the cathode, the current path being then completed throughwinding of magnet I05 to the terminal 4 of the secondary winding oftransformer H9. One half-cycle later in the voltage cycle of the secondphase of the voltage supply, terminal 3 will be negative and terminal 5will be positive whereupon current will then flow from terminal 5,through winding 4-3 of transformer III, and thence to the anode of tubeI01 in which an arc will strike between the anode and cathode thereofwhen the anode reaches a certain positive potential value, the currentpath again being completed through the winding of brake magnet I05 toterminal 4 of the secondary of transformer H9. Now since the terminalsof the various transformers are so numbered as to indicate series aidingconnections, it is apparent that in the first case noted the currentfiows through winding l-2 of transformer -I II while one halfcycle laterthe current flows through winding 6-3 of said transformer, thesequential order of the terminals of the transformer here givenindicating the direction of current flow in each case, it being observedthat the current flow through winding I2 is opposite to that throughwinding 34. Furthermore, windings, I-2, 3--4, 45 and 6--1 of transformerIII are all equal, windings' I-2 and 3-4 being connected, respectively,to the anodes of rectifier tubes I06 and I01 while windings 4-5 and 61are connected, respectively, to the anodes of rectifier tubes I08 andI09. Hence, there will be an alternating current flux set up in the coreof transformer I I I when either pair of thyratrons are rectifying whichwill induce a voltage in winding 8-9 of said transformer that willsupply current to the auxiliary windings l8I of motor H2. The mainwinding H9 of motor H2 will be energized with current from the firstphase of the alternating current supply through the contacts on brakemagnet H05, which operates when the direct current flows through it. Themotor will thus start and run in a direction predetermined by the phasesequence between its two windings.

Now if the grids of tubes Hi6 and I01 are made strongly negative withrespect to the cathodes thereof, and the grids of. tubes I06 and I09 aregiven a potential which is either zero or positive with respect to theircathodes, and, as before, the current at terminal 3 of transformer H9 ispositive, current will flow from terminal 3 on said transformer toterminal 1 of winding 6--1 of transformer II I, through the arc path oftube I09, through the winding of brake magnet I05 and back to terminal 4on the secondary of transformer H9 as already described. It will beobserved that, in this case, the flux produced in the core oftransformer III is the reverse of the flux produced when tube I06 wasconducting since the current which flows through windings 6-1 in the 1to 6 direction is opposite to that which flows through the winding I-2in the I to 2 direction. One half-cycle later current will flow fromterminal 5 on the secondary of transformer M9, which is now positive, toterminal 4 on the winding 3-4-5 of transformer III, terminal 5 to theanode of tube I08, through the arc path thereof and back to the terminal4 of the secondary of transformer II9 through the winding of magnet I05.As before, the flux now produced in the core of transformer III is thereverse of that produced when tube I09 was conducting, so that analternating fiux is set up which induces a voltage in winding B9 thatwill operate the motor II2. But the phase of the voltage, however, isnow shifted 180 degrees from that set up when tubes I06 and I01 wereallowed to conduct.

Hence there is a reversal in the direction of phase sequence and,therefore, a reversal in the direction of rotation of the motor I I2.

It is thus apparent that the motor II2 can be started, stopped andreversed by controlling the grid voltages applied to the tubes I06I0'Ior I-I09, because the supply of current to the windings of the motor,fed from winding -89,

results from the operation of either of the full wave rectifiers I06-401or I08-I09, which also release the brake I63 by operating the brakemagnet I and further closes the circuit at the brake magnet contacts tothe winding of motor I I2, fed from the first phase of the currentsupply by transformer I86.

The grids of the rectifying tubes I06 and I01 are connected,respectively, .to protective resistances I44 and I45 and the grids oftubes I08 and I09 are similarly connected to protective resistances I46and I41. Except when these tubes are required to operate, as moreparticularly described hereinafter, tubes I08 and I09 receive a negativegrid bias from positively grounded battery I13 through the followingpath: battery I13, resistances I23 and I3I, conductor I39 to resistanceI46 and I41, respectively; and tubes I06 and I01 receive a negative gridbias from battery I13, resistances I36 and I35, conductor I24 toresistances I44 and l45, respectively. The cathodes of tubes I06 andI09, inclusive, are grounded as indicated.

The four thyratrons IOI, I02, I03 and I04 are parts of the controlcircuit IIO. These tubes are electrically arranged to operate in asequence and adapted, when operated, to prime the succeeding tube foroperation, which latter tube, when it does operate, extinguishesthepreceding tube that primed it. A common grid battery I81 providesnegative bias to the grid of. each of the tubes through resistances I55and I56 for the grid of tube IOI, through resistances I53 and I54 forthe grid of tube I02, through resistances I5I and I52 for the grid oftube I03 and through resistances I49 and I50 for the grid of tube I04.There are, further, two resistances in series with each of the tubecathodes extending from negative battery supply I13 as follows:resistances I35 and I36 for the cathode of tube IOI; resistances I33 andI34 for the cathode of tube I02; resistances I3I and I23 for the cathodeof tube I03; and resistances I31 and I38 for the cathode of tube I04.The anodes of the tubes IOI, I02, I03 and I04 are all connectedtogether, and may be connected to the negative grounded battery I32through the contacts of relay H4 or to the negative grounded battery I32through the contacts of relay II3, while extinguishing condensers I43,I42, HI and I40 are connected between the re spective cathodes to quenchthe arc of one tube when the succeeding tube operates, as will bedescribed hereinafter.

The function of this control circuit is to apply suitable voltages tothe conductors I24 or I39 so as to influence the operation of rectifiertubes I 06 and I01 or I00 and I09 and thus control the movement of theswitch brush set II5. The operation of the switch depends, naturally,upon the direction of rotation of the motor H2, and since the brush setH5 is normally located midway between the upper and lower terminal banksI2I and I22, the operation of the motor will have to be determined bythe terminal bank in which the calling line appears. This initialdirectional control is accomplished by the operation of either relay H4or relay N3, the former being responsive to a calling line located inthe upper terminal bank I 2| and the latter to a calling line located inlower terminal bank I22.

An inspection of Fig. 2 will make this operation clear. This figureshows the subscriber's line circuit and the relation between the line,the terminal bank and the relay which initiates the switch operations.11' we assume, for example, that the line appears in the upper bank I2I, then the conductors I82, I83, I84 and I86 are connected to a group ofterminals in said bank. These terminals are horizontally aligned and thebrushes of the brush set I I5 which are also horizontally aligned, willengage the various terminals at approximately the same time. In thefigure, the various terminals are shown vertically displaced forconvenience, and the test brush III of brush set H5 is shown in fullbelow the terminal it engages and in broken lines above. The otherbrushes, which are not shown, will, of course, engage theircorresponding terminals at approximately the same time that brush IIIengages the terminal of the bank to which conductor I85 extends.

It will be seen from Fig. 2 that the line circuit comprises the usualline relay I66 and the cutoff relay I65. To the inner contact of theleft set of contacts of the line relay I66 is wired conductor I61 whichextends to the winding of the bank relay I I4. This relay is, of course,commoned to the line relays of all other line circuits terminating onthe upper bank of the switch so that, when any line in the bankinitiates a call, the relay will operate to produce the signal whichcontrols the rotation of the motor II 2 in the direction appropriate tomove the switch brush set I I5 upwards over the terminals of bank I2 I.

Similar circuit arrangements prevail with respect to the lines which areconnected to the terminals of the lower bank II2, bank relay I I3 beingcommoned to all the line relays of the respective line circuits thereinand operating whenever any of said lines call to produce the signalwhich will operate the motor to move the switch brush set II5 downwardsover the terminals of bank I22 in search of the terminals of the callingline.

Thus when subscriber I68 initiates a call by lifting the substationreceiver off the switchhook, a circuit is completed for line relay I66which extends from positive grounded battery through the winding of saidrelay, right contacts of relay I65, over the line loop, to groundthrough the left contacts of relay I65. The line relay, upon operating,closes an obvious circuit to the winding of the bank relay H4 and, overits right contacts, connects ground to the terminal I69 in the bank as amarking condition of the calling line.

Referring to Fig. 1, the operation of bank relay II4, in response to aline initiating a call in the upper bank I 2I, connects ground toconductor I26 which extends to one side of condenser I28 and toresistance I58 to initiate the operation of control circuit H in themanner to be shortly described. Relay II 4, through conductor I I0,further connects positive battery to the anodes of thyratrons IOI, I02,I03 and I04 which are all commoned. It will be observed that the grid oftube IN is maintained at a negative bias by the battery I81 feedingthrough resistances I55 and I56, as already explained, and that themid-points of the cathode series resistances of'one tube, such as tubeIOI, for instance, is connected to the mid-point of the grid biasingresistance of the next tube I62. Hence when relay II4, operates andgrounds conductor I26 and, therefore, one side of condenser I28, astrong positive surge will be created through this condenser which willrender the grid of tube IIII positive and cause it to become conducting.Similarly, if the calling line had been located in the lower bank, theresult of which would have been the operation of relay II3, ground wouldhave been connected to conductor I21 and to one side of condenser I28creating therein a positive surge which would render the grid of tubeI03 positive and cause it to break down and become conducting. However,due to the fact that the mid-point of the cathode resistances I35 andI36 of tube IIII is connected to the mid-point of the grid resistancesI33 and I34 of tube I02, the fact that the tube I 0| becomes conductingand, therefore, produces a current flow through resistances I35 and I36,the voltage drop through resistance I36 will reduce the negative bias onthe grid of tube I 02 to the point just above conducting, that is,primes" the tube so that any slightly positive surge thereafter willcause it to break down and become conducting except as notedhereinafter.

As before stated, conductor I24 is connected to the cathode of tube IOIand to the grids of the rectifier tubes I06 and I01 through resistancesI44 and I45, respectively. When, therefore, tube IOI becomes conducting,the voltage drop in resistances I35 and I36, which will be the sum ofthe voltages of batteries I13 and I32 less the arc drop in the dischargepath of tube IOI, will be applied to conductor I24 and is of a polaritywhich opposes the negative voltage of battery I13. This will bring thevoltage between the conductor I24 and ground to a slightly positivevalue, allowing the rectifier tubes I06 and ID! to conduct and cause themotor II2 to operate and drive the brush II upwards as previouslydescribed.

Similarly, if tube I03 becomes conducting as a result of ground beingapplied to conductor I21, a positive voltage to ground will be appliedto conductor I38, connected to the cathode of tube I03, and this willallow thyratrons I08 and I09 to conduct, causing motor II2 to rotate inthe opposite direction and drive the brush set II5 downwards over theterminals of bank I22.

It is thus seen that the operation ofrelay II 4 starts the switchupwards towards the marked terminal of the calling line by causing tubeIOI to operate and hence allow thyratrons I06 and I07 to conduct therebyreleasing the brake I63 through the operation of magnet I05, and causingmotor II2 to rotate in a counterclockwise direction. When the huntingbrush I II in the brush assembly I I5 engages the marked terminal, whichis grounded, in bank I2I, as shown, for instance, in Fig. 2, a circuitwill be completed from ground on said terminal extending thereto fromthe right contacts of relay I66, brush I'II, conductor I59, the primarywinding of transformer I60, to positive grounded battery "2. The currentwhich builds up in this circuit causes a negative voltage to appearmomentarily at the lower terminal of the secondary winding of saidtransformer, the current produced by which is transmitted throughblocking condensers I14 and I15 to the grids of tubes I02 and I04 whichare thus rendered positive and will strike an are if in a critical orprimed condition. The center tap of the secondary winding of transformerI60 being grounded, a positive pulse is produced tubes IM and I03 whichare thus rendered more negative. Transformer I60 is so proportioned thatthe make or break pulses from its secondary, resulting from the make orbreak of the primary circuit will operate only the tube primed foroperation, as previously explained. Inasmuch as tube I02 is the onewhich was primed by the tube I M when it became conducting, tube I02will operate on the positive pulse transmitted through condenser I15,causing it to apply negative voltage to the grids of tubes I06 and I!whichare thereby disabled to prevent first phase current from beingapplied to the primary windings of transformer III, and to preventrectified current from being applied to the brake magnet lit. The resultis that the motor stops and the brake I63 is released to grip the tapeH6.

The operation of tube I02 causes tube I03 to be primed in the samemanner as tube I02 was itself primed, that is, by applying the voltagedrop through resistance I33 to the grid of tube I03. Tube IIlI, however,is extinguished at the time when tube I02 becomes conducting because,when tube IOI became conducting, a difference of potential wasestablished across each of the two condensers I43 and I40 connected to'the cathode of this tube. But when the impulse through condenser I15causes tube I02 to are, its cathode potential will suddenly rise fromthe potential value prevailing in its primed condition to a valuemeasured by this potential and that of the potential difference in thetwo condensers. Accordingly, a positive potential surge is transmittedby condensers I42 and I43 connected to either side of the cathode oftube I02 which will have the effect of momentarily raising thepotentials of the cathodes of tube IM and I03, which means that thepotential drop between the anode and cathode of tube MI is momentarilylowered below the normal value necessary to maintain the arc.

Tube I03 remains primed until it has been made conducting by asucceeding impulse, or the control circuit is restored to normal by therelease of relay H4 and the consequent removal of battery I32 from theanodes of the tubes.

If the brushset II5 stays on the terminal set. the above describedcircuit operations would complete the hunting. The brush set, however,may have been traveling at such speed that the quantity of kineticenergy stored in the brush, tape, pulleys and motor rotor will cause thetest brush to overrun and break with the terminal, on its upper edge asshown by the broken line indication of brush III in Fig. 2.

Should brush III break with the upper edge of the terminal on this orany other account, a break pulse will be transmitted into the controlcircuit M0 by virtue of the fact that the circuit previously establishedthrough the transformer by the brush III is now broken. This break pulsewill be of opposite polarity to the make pulse and so will operate tubeI 03 which was previously primed, which, in turn, allows tubes I08 andI09 to become conducting, the effect of which will be to reverse therotation of motor II2 to drive brush II5 back towards the groundedterminal, operate magnet I05 to release brake I03, prime tube I04 andquench tube I02. When the brush again engages this terminal, a makepulse will cause the next tube I04 to operate which, in turn, causes thepriming of tube IN and the quenching of tube I03. Should brush III, onits downward travel, break with the bottom edge of the terminal, anotherpulse will be produced which will operate tube IOI, prime tube I02 andextinguish tube I04, causing thereby a reversal of motor operation andan upward travel of the brush, the operations continuing, the brushbeing driven towards the marked terminal after each break pulse. themotor being stopped and the brake being set until the brush comes torest on the marked terminal.

When the tip and ring brushes (not shown) of the brush set H5 haveengaged the corresponding tip and ring terminals of the line in thebank, the cut-off relay IE5 is operated in the well-known manner,thereby releasing line relay I60 andthe bank relay H4. The release ofrelay II4 removes battery supply I32 from the anodes of the controlthyratrons IOI, I02, I03 and I 04 the effect of which is to quench thearc in the thyratron which was last operating and again make negativethe grid of the rectifying thyratron which was then functioning.

While the invention has been illustrated with reference to a high speedselector switch, it is obvious that its principles may be applied toother indexing or positioning controls. For example, the transformer I60might be operated 'in a photocell circuit to permit light beam controlof the brush position, and this control might be used with elevators tosecure leveling at floors.

What is claimed is:

1. The combination with a rectifying device of en electrical network tocontrol said device comprising a plurality of thermionic elementsarranged to be primed for operation in sequence prior to their operationby a succession of impulses and to be disabled by the operation of asucceeding one of said thermionic elements, each alternate one of saidthermionic elements being adapted when operated to control saidrectifying device.

2. The combination with an electrical network which includes a pluralityof thermionic devices adapted to operate in sequence by a succession ofimpulses, of a full wave rectifier comprising a plurality of otherthermionic devices, means for rendering said other thermionic devicesconducting when alternate ones of said plurality of thermionic devicesoperate, a source of alternating current connected to said otherthermionic devices, and an electromagnet responsive to the rectifiedcurrent produced by said plurality of said other thermionic devices.

3. The combination with a motor and a source of operating currenttherefor, of a first element to control the flow of current in onedirection through said motor, a second element to control the flow ofcurrent in the opposite direction through said motor, signaling impulsemeans, a plurality of ionic devices serially disposed for progressiveoperation subsequent to the priming thereof by a succession of impulsesfrom said impulse means whereby one operated device disables apreviously operated device, and. means controlled by alternate ones ofsaid devices when operated in response to a succession of impulses forrendering effective said first and second control elements inalternation to control the fiow of current from said source to saidmotor in the direction determined by the operated control element.

4. The combination with a selector switch having a bank of terminals, atest brush cooperating with said terminals, a motor for advancing saidbrush in a hunting movement over said terminals, a source oi operatingcurrent for said motor and means for applying a marking potential to anyone of said terminals, of an electrical network for controlling theconnection of said source of current to said motor comprising aplurality of thermionic devices arranged to be primed for operation andcaused to operate in rotation by a succession of impulses of alternatepolarity produced by said brush engaging and disengaging a markedterminal to cause reversals of current flowing from said source to saidmotor, each device upon operating priming the next succeeding deviceior. operation and disabling the next preceding device.

5. In a telephone system, a selector switch having a bank of terminalsets, each set including a test terminal, a brush set including a testbrush cooperating therewith, and means for advancing said brush set in ahunting movement over said bank of terminals, means for applying amarking potential to any one of said test terminals, a first thermionicdevice for controlling said brush set advancing means, asecond-thermionic device responsive to the engagement of said test brushwith a marked terminal for disabling said first thermionic device toarrest the hunting movement of said brush set, and a third thermionicdevice responsive to the disengagement of said test brush from saidmarked terminal in the event said brush set overruns said markedterminal for reversing said advancing means to return said brush set tosaid marked terminal.

6. In a telephone system, a selector switch having two banks of terminalsets, each set including a test terminal, a brush set including a testbrush cooperating therewith, normally positioned midway between saidbanks, and means for ad vancing said brush set in a hunting movementover either one of said banks, means for applying a marking potential toany one of said test terminals, a first thermionic device forcontrolling said brush set advancing means, a second thermionic deviceresponsive to the engagement of said test brush with a marked terminalfor disabling said first thermionic device to arrest the huntingmovement of said brush set and a third thermionic device responsive tothe disengagement of said test brush from said marked terminal in theevent said brush set overruns the marked terminal for reversing saidadvancing means to return said brush set to said marked terminal.

7. In a telephone system, a selector switch having a bank of terminalsets, each set including a -test terminal, a brush set including a testbrush cooperating therewith, and means for advancing said brush set in ahunting movement over said bank of terminals; means for applying amarking potential to any one of said test terminals, a first thermionicdevice for controlling said brush set advancing means, a secondthermionic device controlled by said first thermionic device andresponsive to the engagement of said test brush with a marked terminalfor disabling said first thermionic device to arrest the huntingmovement of said brush set, a third thermionic device controlled by saidsecond thermionic device and responsive to the disengagement of saidtest brush from said marked terminal in the event that said brush setoverruns said marked terminal for reversing said advancing means toreturn said brush set to said marked terminal, and a fourth thermionicdevice controlled by said third thermionic device and responsive to thereengagement of said test brush with said marked terminal for arrestingthe return movement of said brush set.

8. In a telephone system, a selector switch having two banks of terminalsets, each set including a test terminal, a brush set including a testbrush cooperating therewith, normally positioned midway between saidbanks, and means for advancing said brush set in a hunting movement overeither one of said banks, means for applying a marking potential to anyone of said test terminals, a first thermionic device for controllingsaid advancing means to move said brush set in a hunting movement overone of said banks, a second thermionic device responsive to theengagement of said test brush with a marked terminal in said bank fordisabling said first thermionic device to arrest the hunting movement ofsaid brush set, a third thermionic device for controlling said advancingmeans to move said brush set in a hunting movement over the other ofsaid banks, and a fourth thermionic device responsive to the engagementof said test brush with a marked terminal in said other bank fordisabling said third thermionic device to arrest the hunting movement ofsaid brush set.

9. In a telephone system, a selector switch having two banks of terminalsets, each set including a test terminal, a brush set. including a testbrush cooperating therewith, normally positioned midway between saidbanks, and means for advancing said brush set, means for applying amarking potential to any one of said test terminals, a first thermionicdevice for controlling said advancing means to move said brush set in aninitial hunting movement over one of said banks or in a return movementover the other of said banks, a second thermionic device responsive tothe engagement of said test brush with a marked terminal in said onebank for disabling said first thermionic device to arrest the huntingmovement of said brush set, a third thermionic device for controllingsaid advancing means to move said brush set in an initial huntingmovement over said other bank or in a return movement over said one bankand a fourth thermionic device responsive to the engagement of said testbrush with a marked terminal in said other bank for disabling said thirdthermionic device to arrest the hunting movement of said brush set, saidfirst and third thermionic devices being responsive to the disengagementof said test brush from said marked terminal in the event that saidbrush set overruns said marked terminal in its initial hunting movementfor controlling said advancing means to return said brush set to themarked terminal.

10. In a telephone system, a selector switch having a panel bank ofterminals and a brush set cooperating therewith, a belt for traversingsaid brush set across said bank, means for driving said belt in onedirection to traverse said brush set in a hunting movement over saidbank of terminals or in the reverse direction to return said brush settowards its normal position,

means for applying a marking potential to any' one of said terminals,thermionic testing means for arresting the hunting movement of saidbrush set when it engages a marked terminal of said bank, and otherthermionic means operated by said thermionic testing means forcontrolling said driving means to reverse the direction of movement 01'said belt to return said brush set to said marked terminal in the eventsaid brush set overruns said terminal in its hunting movement.

11. In a telephone system, a selector switch having a panel bank ofterminals and a brush set cooperating therewith, a belt for traversingsaid brush set across said bank, a pair or pulleys over which said beltruns, means for driving one of said pulleys to traverse said belt in onedirection in a hunting movement, means for applying a marking potentialto any one of said terminals, a thermionic circuit for testing a markedterminal on said bank when said brush set is engaged therewith and forcontrolling said driving means, said thermionic circuit comprising apair of thermionic devices for controlling said driving means and fortesting the lower and upper edges of said marked terminal in the upwardmovement of said switch in the event said brush overruns said terminal,and another pair of thermionic devices for controlling said drivingmeans in the reverse direction and for testing the upper and lower edgesof said marked terminal in the downward movement of said switch.

12. In a telephone system, a selector switch having a panel bank ofterminals and a brush set cooperating therewith, a belt for traversingsaid brush set across said bank, a pair or pulleys over which said beltruns, means for driving one of said pulleys to drive said belt, amagnetic,

clutch operative to free said belt to move said brush set in a huntingmovement over said terminals, means for applying a marking potential toany one of said terminals, thermionic testing means for arresting themovement of said brush set when it engages a marked terminal of saidbank, and other thermionic means operated by said thermionic testingmeans when said brush connects with said marked terminal for disablingsaid magnetic clutch to engage said belt.

13. In a telephone system, a selector switch having two panel banks ofterminals and a brush set cooperating therewith normally positionedmidway between said banks, a belt for traversing said brush set acrosssaid banks, means for driving said belt, a first thermionic controlmeans for causing said belt to be driven in one direction to move saidbrush set in a hunting movement over one of said banks, a secondthermionic control means for causing said belt to be driven in theopposite direction to move said brush set in a hunting movement over theother of said banks, means for establishing an electrical markingcondition on a terminal 01! either bank, means operative in accordancewith the bank in which a marked terminal is located for determiningwhich one or said thermionic control means shall be effective, and meansfor rendering effective said other thermionic control means for responseto the electrical marking condition on said marked terminal.

14. In a telephone system, a selector switch having a panel bank ofterminals and a brush set cooperating therewith, a belt for traversingsaid brush set across said bank, means for driving said belt in onedirection to traverse said brush set in a hunting movement over saidbank of terminals, a thermionic device operable when said brush setengages a marked terminal during its hunting movement, anelectromagnetically controlled brake engageable with said belt andoperable upon the operation of said thermionic device to stop themovement of said belt and said brush set, and another thermionic deviceoperable following the operation of said first thermionic device forreversing the direction of movement of said belt to return said brushset to said marked terminal in the event said brush set overruns saidterminal in its hunting movement.

15. In a telephone system, a selector switch having two panel banks ofterminals and a brush set cooperating therewith normally positionedmidway between said banks, a belt for traversing said brush set acrosseither bank, means for driving said belt, means for applying a markingpotential to any one of said terminals, a first thermionic control meansfor causing said belt to be driven in one direction to move said brushset in a hunting movement over one of said banks, a second thermioniccontrol means for causing said brush to be driven in the oppositedirection to move said brush set in a hunting movement over the other ofsaid banks, and means operative in accordance with the bank in which amarked terminal is located for determining which one of said thermioniccontrol means shall be effective.

WILLIAM H. T. HOLDEN.

